U.S. patent number 8,085,821 [Application Number 12/592,191] was granted by the patent office on 2011-12-27 for light-enhancing device and method based on use of an optically active lasing medium in combination with digital planar holography.
Invention is credited to Vladimir Yankov.
United States Patent |
8,085,821 |
Yankov |
December 27, 2011 |
Light-enhancing device and method based on use of an optically
active lasing medium in combination with digital planar
holography
Abstract
The light-enhancing system of the invention comprises a laser
diode in which a fully reflecting mirror and/or a partially
reflecting mirror of the laser diode is made in the form of digital
planar holography (DPH) incorporating a mode-reorganization
function that decreases divergence and improves brightness of the
output beam of the system by suppressing high-order modes and
gaining low-order modes, or mode. The holographic elements are made
in the form of rectangular grooves that can be manufactured as
binary features reproduced by methods of nanolithography or
nanoimprinting.
Inventors: |
Yankov; Vladimir (Washington
Township, NJ) |
Family
ID: |
44061895 |
Appl.
No.: |
12/592,191 |
Filed: |
November 23, 2009 |
Prior Publication Data
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|
|
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Document
Identifier |
Publication Date |
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US 20110122468 A1 |
May 26, 2011 |
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Current U.S.
Class: |
372/19;
372/20 |
Current CPC
Class: |
G02B
6/4202 (20130101); G03H 1/0408 (20130101); G02B
6/124 (20130101); H01S 5/141 (20130101); G02B
5/32 (20130101); H01S 2301/185 (20130101); H01S
5/02325 (20210101); H01S 2301/18 (20130101); H01S
5/021 (20130101); H01S 5/2036 (20130101); H01S
3/0804 (20130101); H01S 5/024 (20130101); H01S
5/0655 (20130101) |
Current International
Class: |
H01S
3/098 (20060101) |
Field of
Search: |
;372/19,20 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
US. Appl. No. 12/584,891, filed 2009, V. Yankov. cited by other
.
Article "Digital Planar Holography and multiplexer/demultiplexer
with discrete dispersion" of V. Yankov, et al. in SPIE proceedings
series (Conference Active and passive optical components for WDM
communications III : ( Orlando FL, Sep. 8-11, 2003 )). cited by
other .
Article "Digital optical spectrometer-on-chip" published in Appl.
Phys. Lett. 95, 041105 (2009); by S. Babin, et al. cited by
other.
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Primary Examiner: Park; Kinam
Claims
The invention claimed is:
1. The light-enhancing method of the invention comprising the
following steps: providing an optical system that produces a
mode-reorganized optical beam and comprises an output end from
which the mode-reorganized optical beam is sent from the system; a
wide-aperture lasing medium that radiates an optical beam having
multiple modes, has at least one cladding and a core, said core
having a thickness, and a DPH mode reorganizer with a plurality of
nanofeatures in the form of holographic elements having less depth
than the thickness of the core and smaller dimensions than the
half-wavelength of light emitted by the lasing medium, the
nanofeatures being arranged in a pattern that accomplishes a given
function and locally changes the refractive index of the core; and
a mirror selected from a fully reflecting mirror and a partially
reflecting mirror; said multiple modes comprising essentially at
least one low-order mode and a plurality of high-order modes;
forming a resonator from the lasing medium, the DPH reorganizer,
and the mirror selected from a fully reflecting mirror and a
partially reflecting mirror; and bouncing the light emitted from
the lasing medium back and forth in the resonator and reorganizing
the modes by using the given function for gaining said at least one
low-order mode by suppressing the high-order modes, thus increasing
brightness of the light emitted from the system.
2. The method of claim 1, further comprising the step of forming
the grooves as straight grooves that can be produced as binary
features by methods of nanolithography or nanoimprinting.
3. The method of claim 2, wherein the DPH mode reorganizer is used
as a fully reflecting mirror and is located at the closed end of
the optical system, which does not pass the optical beam.
4. The method of claim 2, wherein the DPH mode reorganizer is used
as a partially reflecting mirror and is located in an intermediate
position or at the output end of the optical system.
5. The method of claim 1, further comprising the step of using said
at least one cladding as a lower cladding and using the lower
cladding for propagating the mode-reorganized optical beam to the
output end of the optical system.
6. An optical system that produces a mode-reorganized optical beam,
the optical system comprising: an output end through which a
mode-reorganized optical beam is sent from the system; an optically
active lasing medium that radiates an optical beam having multiple
modes, has at least one cladding and a core, said core having a
thickness; a DPH mode reorganizer with a plurality of nanofeatures
in the form of holographic elements having less depth than the
thickness of the core and smaller dimensions than the
half-wavelength of light emitted by the lasing medium, the elements
being arranged in a pattern that accomplishes a given function and
locally changes the refractive index of the core; and a mirror
selected from a fully reflecting mirror and a partially reflecting
mirror; said multiple modes comprising essentially at least one
low-order mode and a plurality of high-order modes, the optically
active lasing medium, the DPH mode reorganizer, and one of the
mirrors selected from a fully reflecting mirror and a partially
reflecting mirror forming an optical resonator, which has a closed
end that does not pass the optical beam, in said resonator the
light emitted from the lasing medium being bounced back and forth
and the given function being a function of gaining said at least
one low-order mode by suppressing the high-order modes, thus
increasing brightness of the light emitted from the system.
7. The optical system of claim 6, wherein the resonator has a
closed end and wherein the DPH mode reorganizer comprises the fully
reflecting mirror installed at the closed end of the resonator.
8. The optical system of claim 6, wherein the DPH mode reorganizer
comprises a partially reflecting mirror installed at the output end
of the system.
9. The optical system of claim 6, wherein said at least one
cladding is a lower cladding that receives the optical beam from
the core of the lasing medium and through which the
mode-reorganized beam propagates to the output end of the
system.
10. The optical system of claim 9, wherein the resonator has a
closed end and wherein the DPH mode reorganizer comprises the fully
reflecting mirror installed at the closed end of the resonator.
11. The optical system of claim 9, wherein the DPH mode reorganizer
comprises a partially reflecting mirror installed at the output end
of the system.
12. The optical system of claim 11, wherein the holographic
elements are grooves.
13. The optical system of claim 12, wherein the grooves are
rectangular for use as binary features reproduced by methods of
nanolithography or nanoimprinting.
14. An optical system that produces a mode-reorganized optical
beam, the optical system comprising: an output end through which a
mode-reorganized optical beam is sent from the system; a laser
diode comprising an optically active lasing medium that radiates an
optical beam having multiple modes comprising essentially at least
one low-order mode and a plurality of high-order modes, has at
least one cladding and a core, said core having a thickness, and at
least one mirror selected from a fully reflecting mirror and a
partially reflecting mirror; and a DPH mode reorganizer with a
plurality of holographic elements having less depth than the
thickness of the core and smaller dimensions than those of the
half-wavelength of light emitted by the lasing medium, the elements
being arranged in a pattern that accomplishes a given function and
changes the refractive index of the core, said DPH mode reorganizer
comprising said at least one mirror, in said resonator the light
emitted from the lasing medium being bounced back and forth and the
given function being a function of gaining said at least one
low-order mode by suppressing the high-order modes, thus increasing
brightness of the light emitted from the system.
15. The optical system of claim 14, wherein said multiple modes
comprising essentially at least one low-order mode and a plurality
of high-order modes, the optically active lasing medium, and the
DPH mode reorganizer forming an optical resonator that has a closed
end, which does not pass the optical beam; and the given function
being a function of gaining said at least one low-order mode by
suppressing the high-order modes, thus increasing brightness of the
light emitted from the system.
16. The optical system of claim 15, wherein the resonator has a
closed end and wherein the DPH mode reorganizer comprises the fully
reflecting mirror installed at the closed end of the resonator.
17. The optical system of claim 14, wherein the DPH mode
reorganizer comprises a partially reflecting mirror installed at
the output end of the system.
18. The optical system of claim 14, wherein said at least one
cladding is a lower cladding that receives the optical beam from
the core of the lasing medium and through which the
mode-reorganized beam propagates to the output end of the
system.
19. The optical system of claim 18, wherein the resonator has a
closed end and wherein the DPH mode reorganizer comprises the fully
reflecting mirror installed at the closed end of the resonator.
20. The optical system of claim 18, wherein the DPH mode
reorganizer comprises a partially reflecting mirror installed at
the output end of the system.
Description
FIELD OF THE INVENTION
The present invention relates to a light-enhancing device and a
method based on the use of laser diodes in combination with digital
planar holography. More specifically, the invention relates to a
light-enhancing device and a method based on the use of a multimode
laser diode in combination with digital planar holography to reduce
the number of modes with a resulting decrease in divergence,
without a noticeable loss of optical light power, and with a
resulting improvement of brightness. The device may find use for
transmitting light between various optical systems and for
improving brightness of the output beam emitted from the system.
The device may also be used as a source of light that has high
intensity even in far-field regions.
BACKGROUND OF THE INVENTION
One of the problems that currently exists in the field of laser
technology is insufficient quality of radiation from edge-emitting
laser diodes. It is known that light beams emitted from
edge-emitting laser diodes have a complicated structure. These
beams are asymmetric and exhibit different divergences in the
emitter plane (slow axis) and in the plane perpendicular to the
emitter plane (fast axis). Although fast-axis divergence is much
greater, the wavefront is close to a diffraction-limited spherical
shape, and the beam can be easily collimated with a spherical lens.
On the other hand, the slow-axis beam structure can be very
complicated, and collimation is very difficult, if even
possible.
The radiation structure of the aforementioned type significantly
complicates formation of desired beams and their collimation and
focusing on a target, as well as coupling into optical fibers. A
common solution to the above problem demands the use of anamorphous
optics, such as special collimators for fast and slow axes, special
focusing optics, etc. However, precision collimators of this type
are expensive, and this limits their use in practice. Designs of
collimators for beams propagated in the direction of slow and fast
axes are known and described in numerous patents, for example, U.S.
Pat. Nos. 4,687,285; 5,940,564; 6,031,953, and European Patent EP
No. 864,892.
It is understood that the above problem is even greater for laser
diodes with wide emitters, i.e., with emitters having a high ratio
of emitter width to emitter height. The driving force behind
widening the emitter area is the desire to increase output power
without damaging the output face of the laser diode. An example of
such laser diodes that recently appeared on the market is a device
having an emitter width greater than 100 microns (slow axis) and a
height of less than 1.5 microns (fast axis). The output power of
these diodes exceeds several watts and may reach tens of watts, and
the structure of their radiation has a complicated multimode nature
that leads to high divergence of the output beam. Conventional
approaches to the solution of the above problem with respect to the
wide-aperture edge-emitting laser diodes do not allow for forming
single transverse mode beams without significant loss of power and
increase in weight or size. Therefore, advantages inherent in
optical laser devices are not used to their full potentials for
wide-aperture edge-emitting laser diodes.
In view of the above, the problem of improving optical
characteristics such as mode composition, beam divergence in the
direction of slow and fast axes without noticeable reduction in
optical power, and, hence, brightness, is an extremely important
task in laser technology.
An innovative method of controlling the optical parameters of a
light beam such as direction of light propagation, change of
phases, spectral dispersion, etc., as proposed in U.S. patent
application Ser. No. 12/011,453 filed Jan. 28, 2008, is the use of
planar optical waveguides with quasi continuous change in the
refractive index. This approach is known as digital planar
holography (DPH), a new technology recently developed for
fabricating miniature components for integrated optics. The essence
of DPH technology is the embedding of digital holograms calculated
by a computer inside a planar waveguide.
The DPH allows for light propagation in the hologram plane rather
than in the perpendicular direction and results in a long
interaction path. Benefits of a long interaction path are well
known for volume/thick holograms. On the other hand, planar
configuration provides easy access to the surface, where the
hologram should be embedded, enabling a simple fabrication
process.
As known, light is confined in waveguides by a refractive index
gradient and propagates in a core layer surrounded with a cladding
layer. Materials for core/cladding layers should be selected so
that the core refractive index N.sub.core is greater than that of
the cladding layer N.sub.clad: N.sub.core>N.sub.clad.
Cylindrical waveguides (optical fibers) allow for one-dimensional
light propagation along the axis. Planar waveguides, which are
fabricated by sequentially depositing flat layers of transparent
materials with a proper refractive index gradient on a standard
wafer, confine light in one direction (axis z) and permit free
propagation in two other directions (axes x and y).
A lightwave propagating through the waveguide core extends to some
degree into both cladding layers. If the refractive index is
modulated in the wave path, the light from each given wavelength
can be directed to a desirable point.
DPH technology can be used for designing and fabricating
holographic nanostructures inside a planar waveguide, thus
providing conditions for light processing and control. There are
several ways of modulating the core refractive index, the simplest
of which is engraving the required pattern by means of
nanolithography. Modulation is created by embedding a digital
hologram on one of the core/cladding interfaces or on both of them.
Standard lithographical processes can be used, making mass
production straightforward and inexpensive. Nanoimprinting is
another viable method for fabricating DPH patterns. Each DPH
pattern is computer-generated and is customized for a given
application. The consists of numerous nanogrooves, each .about.100
nm wide, positioned so as to provide maximum efficiency for a
specific application.
The devices are fabricated on standard wafers. While the total
number of nanogrooves is huge (=10.sup.6), the typical size of DPH
devices is on a scale of millimeters.
DPH structure can be described as a digital planar hologram that
comprises an optimized combination of overlaid virtual subgratings,
each of which is resonant to a single wavelength of light.
SUMMARY
The light-enhancing device of the invention comprises a lasing
medium installed on a substrate sub mount made, e.g., of silica,
and a DPH mode reorganizer, which is formed on a standard wafer
substrate according to specific application of the device. Both
units are supported by a mounting plate that also can be made of a
suitable material of high thermal conductivity, such as ceramic
having high thermal conductivity.
The optically active lasing medium (wide-aperture emitter) radiates
a multimode light beam. In this context "wide-aperture" means that
its width ranges from 10 microns to several hundred microns. The
height of the emitter ranges from 0.2 nm to several microns. The
active lasing medium is limited on one side with a fully reflective
mirror and on the other side with antireflective coating having a
very low reflection coefficient (R<0.1%).
The DPH mode reorganizer is supported by a silicon substrate and
comprises a cladding layer that rests on the silicon substrate and
comprises the following: (1) a layer of a lower cladding of
SiO.sub.2 having a thickness ranging from several to several tens
of microns; and (2) a core placed onto the lower cladding that is
made of silicon doped with a material such as germanium, which
increases the core refractive index, and having a thickness of
several nanometers to one micron. The upper cladding and core have
different refraction indices that differ by 1 to 5%. In other
words, the refraction index of the core is greater than that of the
cladding. If necessary, an upper cladding can be deposited onto the
core.
The core of the DPH unit comprises a plurality of holographic
elements (hereinafter referred to as "elements") that can be
produced in the form of grooves with a depth less than the
thickness of the core. Preferably, the holographic elements are
manufactured as rectangular grooves reproduced by methods of binary
nanolithography. The number of such elements can exceed 10.sup.6.
The total surface area occupied by these elements on the surface of
the core is several mm.sup.2. The elements locally change the
refractive indices of the core. It is understood that if the
dimensions of the elements do not exceed half of a light
wavelength, the density of the elements on the core surface can be
used for controlling propagation of the light beam. This means that
the light beam emitted from the lasing medium can be converted,
after passing through and processing inside the DPH unit, into a
beam of desired parameters defined by the DPH structure and
configuration.
Both the lasing medium and the DPH mode reorganizer can be mounted
on a common base plate made, e.g., of Si, SiO.sub.2, or quartz. To
stabilize temperature in lasing media of high power, the common
base plate can be made from a material of high thermal conductivity
on a thermoelectric cooler. The lasing medium and the DPH mode
reorganizer are mounted on the base plate so that the optical axis
of the lasing medium is aligned with the optical axis of the core
and the respective axis of the hologram, e.g., the symmetry
axis.
In a conventional wide-aperture lasing medium without use of the
above-described DPH mode reorganizer, the output beam will have a
multimode nature that consists of several tens or even hundreds of
various transverse lateral modes of various intensities.
The picture dramatically changes when the lasing medium is
optically coupled with the specific DPH mode reorganizer of the
invention because the DPH mode reorganizer decreases the number of
modes to one, two, or three. As a result, a powerful low-order mode
is formed, and the major part of the power output of the laser is
concentrated in this low-order mode, while a much smaller part of
the laser power is held by the remaining side modes, the total
number of which is significantly reduced to one, two, or three.
This mode distribution pattern is typical for the far field.
Angular divergence in the direction of the slow axis can be reduced
in order of magnitude, e.g., from 20.degree. to 2.degree..
It was unexpectedly discovered that with use of the DHP-mode
reorganizer in the present invention, divergence in the direction
of the fast axis was also reduced, in this case by a factor of 4,
i.e., from approximately 40.degree. to 10.degree.. Also, it was
discovered that the beam that was collimated in the core during
propagation through the DPH mode reorganizer was then transferred
to the lower cladding, at which point it was sent from the
system.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a block diagram of a conventional optical system with a
multimode lasing medium and with collimating optics.
FIG. 1B is a block diagram of the mode-reorganizing optical system
of the invention, wherein the DPH mode reorganizer is located
between the laser-active medium and the output optics and is used
as a partially reflecting mirror.
FIG. 1C is a block diagram of the mode-reorganizing optical system
of the invention, wherein the DPH mode reorganizer functions as a
fully reflecting mirror and is located at the closed end of the
resonator.
FIGS. 2A, 2B, 2C, 2D, 2E, and 2F are simplified schematic top views
of the mode-reorganizing system according to the invention that
illustrates interaction of the selected modes of the laser with
respective subgratings, which, in reality, are combined into a
single supergrating.
FIG. 2G is a simplified top view of the system of the invention
that shows a lasing medium and a supergrating composed of
subgratings of the types shown in FIG. 2A through FIG. 2F.
FIG. 3 is a side sectional view of a light-enhancing device
according to one aspect of the invention, wherein the DPH mode
reorganizer is used as a partially transparent mirror.
FIG. 4 is a top view of the device shown in FIG. 3.
FIG. 5 is a graph illustrating angular dependence of
light-intensity distribution in the far field for the conventional
system shown in FIG. 1.
FIG. 6 is a graph illustrating angular dependence of
light-intensity distribution in the far field for the system of the
invention (shown in FIGS. 3 and 4), which incorporates the DPH mode
reorganizer.
FIG. 7 is a side sectional view of the light-enhancing device
according to another aspect of the invention, wherein the DPH mode
reorganizer is used as a fully reflecting mirror.
DETAILED DESCRIPTION OF THE INVENTION
Terminology used in the present specification is explained below.
In the context of the present patent specification, the term
"lasing medium" relates to a part of a laser type of light-emitting
device, such as a laser diode, that forms the aforementioned device
in combination with respective fully reflecting and/or partially
reflecting mirrors.
Furthermore, although mode structures are considered in general,
all of the modifications considered below relate to lateral modes.
Some important properties of laser diodes depend on the geometry of
the optical cavity. Thus, in the vertical direction, light is
contained in a very thin layer, and, therefore, structure supports
only a single optical mode in the direction perpendicular to the
layers. However, in the lateral direction, if the waveguide is wide
when compared to the wavelength of light, then the waveguide can
support multiple lateral optical modes, and the laser is known as
"multimode."
FIG. 1A is a block diagram of a conventional optical system 20a
with a multimode wide-aperture laser diode 22a and with collimating
optics 24a. The multimode laser diode 22a comprises a lasing medium
26a and is located between a partially reflecting output mirror 30a
and a fully reflecting mirror 28a. In the structure of the
multimode laser diode 22a, the mirrors 28a and 30a form an optical
resonator 32a. Reference numeral 25a designates an emitter of the
laser diode 22a located on the outer side of the partially
reflecting mirror 30a. All of these parts are mounted on a submount
S. In the resonator 32a, the light applied from the laser-active
medium 26a bounces back and forth between the mirrors 28a and 30a,
enhancing stimulated emission. The beam B that is sent from the
laser diode 22a is collimated by the optics 24a, thus producing a
collimated output beam B1 (FIG. 1A).
The system 20a described above and shown in FIG. 1A is a well-known
structure used in a lasing medium technique. The inventor herein
has found that in addition to light reflection, one or both mirrors
28a and/or 30a can impart to the system 20a a new function, i.e.,
the function of an optical-mode reorganizer, which provides
intercoupling of all modes of the beam of a lasing medium 26a in
order to stabilize the radiating wavelength, to synchronize phases,
and to reorganize mode structure by suppressing the high-order
modes and gaining the low-order modes with lower divergence or even
a single mode with the lowest divergence, thus increasing
brightness of the output beam B.
Furthermore, according to the invention, system components used to
accomplish the aforementioned beam-reorganizing function and other
functions can be embodied as DPH optical components formed as
digital holograms generated in a computer and embedded into planar
waveguides by standard mass-production methods, such as binary
nanolithography or nanoimprinting. As a result, a mode-reorganizing
system 120a of the type shown in FIG. 1B is obtained. In the
context of the present invention, the term "mode-reorganizing
system" covers an assembly comprising an optically active lasing
medium, a planar waveguide that contains a DPH mode reorganizer,
and at least one mirror (fully or partially reflecting).
In the mode-reorganizing system 120a of the invention, the
mode-reorganized optical-beam components that are similar to those
shown in FIG. 1A are designated by the same reference numerals with
the addition of 100. For example, the system 120a comprises a
lasing medium 126a and an output optical unit 124a that manages the
beam B2 emitted from the lasing medium 126a and produces the output
beam B3 (FIG. 1B) from an output end 127a of the system. In
distinction from the conventional system 20a of FIG. 1A, the system
120a incorporates a mode reorganizer 130a made in the form of a DPH
device, hereinafter referred to as "DPH mode reorganizer." The
lasing medium 126a is placed on the same basic support plate Sa on
which the DPH mode reorganizer 130a that accomplishes the
aforementioned new functions (i.e., the functions of reorganizing
modes of the multimode laser) is placed in order to narrow the
radiating space spectrum, to improve synchronization of the mode
phases of all modes, and to reorganize the modes so that the side
modes are suppressed while the low-order modes, or mode, are
gained. In other words, after integration on a support plate Sa,
the laser-active medium 126a and the DPH mode reorganizer 130a form
a single optical chip. Optical coupling and interaction between the
lasing medium 126a and the DPH mode reorganizer 130a is carried out
through a well-known method of optical butt connection between two
laser media.
The system 120a also incorporates a fully reflecting mirror 128a.
The lasing medium 126a, the DPH mode reorganizer 130a, and the
fully reflecting mirror 128a form an optical resonator 132a. In the
resonator 132a, the light applied from the laser-active medium 126a
bounces back and forth between the DPH mode reorganizer 130a and
the fully reflecting mirror 128a, thereby enhancing stimulated
emission. Thus, in addition to its main function of mode selection,
the DPH mode reorganizer 130a accomplishes the function of a
partially reflecting mirror.
In addition to the above, the DPH mode reorganizer 130a
participates in bouncing of the light beam back and forth and in
aforementioned reorganization of the modes by suppressing the side
modes and gaining the low-order modes, or mode. As a result, output
radiation acquires coherency and increased brightness due to
decrease of the spatial divergence.
Alternatively, as shown in FIG. 1C, the coherent-beam system 120b
of the invention can be embodied so that the DPH mode reorganizer
130b accomplishes the function of a fully reflecting mirror. In
general, this system is the same as that shown in FIG. 1B but
differs from system 120a in that this system includes the partially
transparent mirror 131b located between the lasing medium 126b and
the DPH mode reorganizer 130b, and the DPH mode reorganizer 130b is
used instead of the fully reflecting mirror 130a. Other components
of the system 120c, which are similar to the components of the
system 120b and accomplish the same functions, are designated by
the same reference numerals but are accompanied by addition of the
letter "b" instead of the letter "a". The beam B4 sent from the
emitter of the lasing medium 126b is sent to a collimation optics
124b that emits an output beam B5 of improved brightness. The
symbol Sb designates a submount. In the system 120b, the DPH mode
reorganizer 130b, the optically active medium 126b, and the
partially transparent mirror 131b form the optical resonator 122b
wherein the DPH mode reorganizer functions as a fully reflecting
mirror and is located at the closed end of the resonator, which
does not pass the optical beam.
In operation, the multimode beam propagates back and forth through
the active optical medium 126b between the DPH mode reorganizer
130b and the partially transparent mirror 131b. In this process,
the modes are reorganized, with suppression of the high-order modes
and enhancement of the low-order modes.
In both systems 120a and 120b, each DPH mode reorganizer 130a and
130b, respectively, has a complicated hierarchical structure, which
in approximation can be considered substantially as a supergrating
consisting of standard binary nanofeatures (for example, etched
grooves of a rectangular shape) formed in a planar waveguide in
order to modulate its effective refractive index. Each binary
nanofeature is defined by three dimensions: width, length, and
depth. The width and depth of these nanofeatures are shorter than
the laser wavelength of radiation interacting with the
aforementioned grooves.
As a light beam is confined inside the planar waveguide, it is
forced to propagate through and optically interact with the DPH
structure, which results in mode reorganization with the
aforementioned reorganizing that leads to suppression of the
high-order modes and hence increase in brightness of the output
beam emitted from the system.
As mentioned above, all optical components in systems 120a and 120b
are implemented as integrated devices in the form of planar optical
chips. Optionally, there can be one planar chip for the laser
active media 126a and 126b and the DPH mode reorganizers 130a and
131b together, or two planar chips, i.e., one for the laser media
and another for the DPH mode reorganizers. In the second case, both
chips are optically coupled with each other.
According to the present invention, each DPH mode reorganizer is
implemented as a combination of holographic elements, e.g.,
nanogrooves embedded into a planar waveguide for periodical
modulation of its refractive index. The modulating function is
calculated based on optical-transfer functions, desirable in said
reorganizer and implemented by standard mass-production methods
such as nanolithography or nanoimprinting. Numerous nanofeatures
(e.g., in an amount of 10.sup.5-10.sup.6) can be aggregated into
multiple subgratings, each of which is responsible for an
optical-transfer function.
Each subgrating is a group of DPH features specifically selected to
accomplish a predetermined function from multiple functions of the
DPH mode reorganizer. All subgratings are superimposed on the same
planar area, forming a supergrating that performs all desired
functions.
Each supergrating is generated as a mathematical superposition of
elliptic, parabolic, or hyperbolic subgratings, with a spatial
period of an approximate one-half wavelength according to the
following method. The first to be created is a two-dimensional
analog-generating function A(x,y) representing a superposition of
modulation profiles of the refractive index. Each modulation
function corresponds to the equivalent of a subgrating. Determined
in this step is a two-dimensional-generating function A(x,y) that
resembles the profile of a refractive index in a planar waveguide
corresponding to desired optical transfer functions to be
implemented.
The next step is binarization of the two-dimensional
analog-generating function A(x,y), which was produced in the
previous step. Binarization is achieved by applying a threshold
value and assigning 1 to all areas above the predetermined
threshold and 0 to the remaining areas in order to obtain a
digital, two-dimensional-generating function B(x,y).
Next, the complex shape islands in B(x,y) with the value of 1 are
simplified for presentation as a combination of standard
microlithographic or nanolithographic features (short and straight
grooves). This is accompanied by conversion to function C(x,y).
The last step is lithographic fabrication of the standard
nanofeatures by microlithography or nanoimprinting as function C(x,
y) to a calculated depth on a planar waveguide.
An example of the structure of a mode-reorganizing system according
to the invention is shown in FIGS. 2A to 2F. For clarity, each
drawing illustrates interaction of the selected modes of the laser
with respective subgratings, which, in reality, are combined into a
single supergrating. In other words, in each respective drawing,
the illustrated system, which as a whole is designated by reference
numeral 200a, consists of a multimode laser 220 and respective
subgratings selected from subgratings 222a through 222f.
The lasing medium of a multimode laser 220 emits a multimode beam
221 that has a complicated structure consisting of several modes.
The output beam can be presented as a combination of subbeams
radiated by narrow, active regions or (that form the lasing
medium). The width of these regions is selected so that the subbeam
from each region is a single-mode beam.
The subbeams 220a and 220b of the multimode beam 221 propagate to
the supergrating and are reflected by subgrating 222a in a planar
waveguide so that the corresponding regions or subvolumes 1a and 2b
of the active medium are intercoupled (FIG. 2A). As a result, the
subvolumes emit single-mode beams which would not be of the same
mode if they were not intercoupled, but intercoupling forces them
to generate the same mode of radiation. Ideally, the output beam
will have the same parameters as from one single-mode region, but
its power will be doubled. Accordingly, the brightness of the
combined intercoupled beam is also doubled. In reality, it will not
be a factor of two because of inevitable losses associated with
intercoupling, but the enhancement factor will be close to 2
because the losses can be minimized by using a high-transparency
waveguide and low-loss butt-coupling.
As known, a single-mode laser resonator should satisfy the
following condition: a.sup.2/(.lamda.*L)<1, (1) where a is the
laser aperture, .lamda. is radiation wavelength (inside the
resonator), and L is the resonator length.
In accordance with formula (1), the size of the single-mode region
in the wide-aperture laser diode can be calculated. The total
number of single-mode regions N can be estimated as: N=A/a, (2)
where A is the width of a wide-aperture active medium, and
a<(.lamda.*L).sup.0.5 (3)
For typical values of parameters a and A, the number of single-mode
subregions ranges from N.about.3/30. Parameter N determines the
number of subgratings to be superimposed to form the
supergrating.
Consider, for example, an imaginary laser that radiates a
three-mode beam, wherein each mode corresponds to a subbeam. FIGS.
2A, 2B, 2C, 2D, 2E, and 2F illustrate operation of a system that
contains a grating consisting of six subgrating 222a, 222b, 222c,
222d, 222e and 222f, respectively, shown in separate drawings. In
FIG. 2A, the subbeam 220a corresponds to a region 1a, and the
subbeam 220b corresponds to a region 2b. For simplicity in the
drawings, designations of subbeams in FIGS. 2B, 2C, 2D, 2E, and 2F
are omitted but are shown in Table 1, which lists the intercoupling
of subbeams with each other in the resonator (not shown in FIGS. 2A
to 2F)
TABLE-US-00001 TABLE 1 Subbeams Intercoupled by Corresponding
Subgratings subgrating 222a intercouples subbeams 220a and 220b
222b 220a and 220c 222c 220b and 220c 222d 220a to itself 222e 220b
to itself 222f 220c to itself
All subgratings are superimposed on the same planar area, forming a
supergrating, where each feature works toward the best synergistic
performance of all desired functions. In general, for the structure
shown in FIG. 2G with supergrating 222N and N single-mode regions,
generating single-mode subbeams 220a', 200b', and 220c' through
220n', the required number of subgratings can be calculated in the
following way: subbeam 220a' needs to be coupled with N modes
(including itself), subbeam 220b' needs to be coupled with N-1
modes because it has already been coupled with mode 220a', subbeam
220c' needs to be coupled with N-2 modes because it has already
been coupled with modes 220a', 220b', and so on; finally, subbeam
220n' needs to be coupled with itself only because it has already
been coupled with all other subbeams.
Therefore, the total number of subgratings m is the sum of the
arithmetic progression: m=N+(N-1)+(N-2)+ . . . +1, (4) i.e.,
m=0.5N(N+1) (5)
As mentioned, a DPH mode reorganizer can be used as a fully
reflecting resonator mirror or as a partially reflecting mirror.
Such modifications can be provided by varying the length of the DPH
mode reorganizer; short supergratings reflect only partially, and
the reflection coefficient grows with structure length and after
becoming saturated does not depend on additional increase in
length, i.e., forms a fully reflecting component.
As follows from the above formula (5), the number of subgratings
grows in arithmetic progression with increase in the size of the
laser aperture.
FIG. 3 is a sectional view of a light-enhancing device of the
invention, which, as a whole, is designated by reference numeral
300. FIG. 4 is a top view of the device shown in FIG. 3. The device
300 comprises a lasing medium 302 installed on a substrate submount
304 made of, e.g., silicon, and a specific DPH mode reorganizer
306, which is formed on a silicon substrate 308 according to
specific application of the device. Both units 302 and 306 are
supported by a mounting plate 310 that also can be made of a
suitable material of high thermal conductivity, such as
ceramic.
In combination with mirrors, as described below, the lasing medium
302 radiates a multimode light beam and has a relatively wide
emitter 312 with a width ranging from about 10 microns to several
hundred microns. The height of the emitter ranges from about 0.2 nm
to several microns. The active lasing medium 302 (FIG. 4) is
limited on one side with a fully reflective mirror 316 and on the
other side with an antireflective coating 318 having a very low
coefficient of reflection (R<0.1%).
The DPH mode reorganizer 306 is supported by a silicon substrate
308 and comprises (1) a lower cladding layer 320 (e.g., of
SiO.sub.2) that rests on the silicon substrate 308 and has a
thickness in the range of several to several tens of microns, and
(2) a core 322 that is placed onto the lower cladding, which is
made of SiO.sub.2 doped with a material such as germanium which
changes the refraction index by 1 to 5%, and has a thickness of
about several nanometers to one micron or is doped with any other
transparent material having a refractive index greater than that of
the cladding (FIG. 3). If necessary, an upper cladding with a
refractive index lower than that of the core can be placed onto the
core 322.
The core 322 of the DPH mode reorganizer 306 comprises a plurality
of holographic elements, hereinafter referred to as "elements," in
the form of grooves 324 with a depth of less than the thickness of
the core. Preferably, the elements are rectangular grooves produced
as binary nanofeatures suitable for reproduction by methods of
nanolithography or nanoimprinting.
The number of such elements can exceed 10.sup.6. The total surface
area occupied by these elements on the surface of the core is
several mm.sup.2. An example of this pattern of elements is shown
in FIG. 4. The elements, or grooves 324, locally change the
refractive indices of the core. If the dimensions of the elements
do not exceed the dimensions of the wavelength, the density of the
elements on the core surface can be used to control the density of
the light beam. This means that the light beam B5, B6, and B7 (FIG.
4) that enters the DPH mode reorganizer 306 from the lasing medium
302 and is processed by the DPH mode reorganizer 306 is converted
into a beam B8 having desired parameters defined by the specific
application of the device 300 as a whole.
As mentioned above, the lasing medium 302 and the DPH mode
reorganizer 306 are both mounted on a common base plate 310 made
of, e.g., Si, SiO.sub.2, or quartz. To stabilize temperature in
laser media of high power, the common base plate 310 can be made
from a material of high thermal conductivity on a thermoelectric
cooler. The lasing medium 302 and the DPH mode reorganizer 306 are
mounted on the base plate 310 so that the optical axis X-X of the
lasing medium 302 is aligned with the optical axis of the core and
the respective axis of the hologram, e.g., the symmetry axis X1-X1.
The light beams, B6, and B7 enter the DPH mode reorganizer 306 from
the lasing medium 302 through an optical glue or an optical gel
layer 326 with a refractive index close to that of the core
322.
In a conventional lasing medium having the above-described geometry
with a fully reflecting mirror and a partially (10%) reflecting
mirror instead of the above-described DPH mode reorganizer 306, the
output beam B1 (FIG. 1A) will have several tens or even hundreds of
different modes that have different intensities. Only some of these
modes, e.g., several to ten modes will have approximately the same
intensity. Therefore, in the far field, the beams B1 emitted by the
laser, such as the laser diode 22a in FIG. 1A, will have
significant divergence in the direction of the slow axis and wide
distribution of radiation consisting of a large number of different
modes. On the other hand, divergence in the direction of the fast
axis may reach even several tens of degrees or more.
The situation dramatically changes when the partially (10%)
reflecting mirror or the fully reflecting mirror is replaced with
the specific DPH mode reorganizer 130a of the invention, as
described above. This situation occurs because the DPH mode
reorganizer 130a converts the multimode beam into a beam with far
fewer space modes. This is shown in FIGS. 5 and 6, which illustrate
the angular dependence of light-intensity distribution in the far
field. FIG. 5 corresponds to the system of FIG. 1A, and FIG. 6
corresponds to the system of FIGS. 3 and 4 for the same lasing
medium as in FIG. 1A but in combination with the DPH mode
reorganizer 130a of the invention.
The proposed invention was experimentally tested with a
wide-aperture laser diode, as described below.
The radiation field of the wide-aperture laser diode 22a in the
far-field region is shown in FIG. 5. It has a multimode structure
with 8 to 10 modes having an angular divergence of about 20.degree.
in the direction of the slow axis.
On the other hand, it can be seen from FIG. 6 that coupling with
the DPH mode reorganizer 130a (FIGS. 1B, 3, and 4) dramatically
changes the structure of the output beam; a powerful low-order mode
is formed, and the major part of the output light power of the
laser is concentrated in this low-order mode, while a much smaller
part of laser power is held by two side modes. Therefore, the total
number of modes is significantly reduced to three and can be even
further reduced to a single-mode structure after refining the
design of the DPH mode reorganizer. For the system of the
invention, such mode-distribution pattern is typical for the far
field. Angular divergence in the direction of the slow axis is
reduced four-fold, i.e., from about 20.degree. to about
5.degree..
It was unexpectedly discovered that divergence in the direction of
the fast axis was also reduced, in this case by a factor of 4,
i.e., from about 40.degree. to about 10.degree.. It was also
discovered that the beams B6 and B7 (FIG. 4) that were collimated
in the core 322 during propagation through and interaction with the
DPH mode reorganizer 306 was then transferred to the lower cladding
320 where it was sent from the system 300 as beam B8, which
possesses the above-mentioned characteristics.
FIG. 7 is a sectional view of a light-enhancing device according to
another aspect of the invention. The device, as a whole, is
designated by reference numeral 400. In general, the device shown
in FIG. 7 is similar to the device 300 shown in FIG. 3 except that
a specific DPH mode reorganizer 406 is configured as a fully
reflective mirror, while a partially reflecting mirror 416
functions as an emitter of the lasing medium 402, which is
supported by a submount 404. An antireflective coating 418 is
placed between the DPH mode reorganizer 406 and the lasing medium
402. The lasing medium 402, the DPH mode reorganizer 406, and the
partially reflecting mirror 416 form a resonator.
During operation of the device 400, the light emitted from the
lasing medium 402 bounces back and forth in the resonator between
the partially reflecting mirror 416 of the lasing medium 402 and
the DPH mode reorganizer 406, which is configured as a fully
reflecting mirror with the function of gaining the low-order modes,
or mode, by suppressing the side modes, thus increasing brightness
of the light beam B9 emitted from the system.
The light-enhancing method of the invention comprises the following
steps: providing an optical system comprising a lasing medium that
radiates multiple modes, has at least one cladding and a core, said
core having a thickness, a DPH mode reorganizer with a plurality of
holographic elements in the form of grooves having less depth of
thickness than that of the core and smaller dimensions than those
of the wavelength of light emitted by the lasing medium, the
elements being arranged in a pattern that accomplishes a given
function and changes the refractive index of the core; and a mirror
selected from a fully reflecting mirror and a partially reflecting
mirror; said multiple modes comprising essentially at least one
low-order mode and a plurality of side modes; forming a resonator
from the lasing medium, the DPH reorganizer, and the mirror
selected from a fully reflecting mirror and a partially reflecting
mirror; bouncing the light emitted from the lasing medium back and
forth in the resonator, and reorganizing the modes by using the
given function for gaining said low-order mode by suppressing the
side modes, thus decreasing beam divergence and increasing
brightness of the light emitted from the system.
Although the invention has been shown and described with reference
to specific embodiments, these embodiments should not be construed
as limiting the areas of application of the invention and any
changes and modifications are possible provided that these changes
and modifications do not depart from the scope of the attached
patent claims. For example, the system cannot contain mirrors but
instead can contain two DPH beam combiners functioning as the
respective mirrors. Replacement of all mirrors with the DPH
structures of this invention applies to all combinations of the
system examples described above.
* * * * *